Article's followed by thank-you letter (5)
I believe the answer is:
theta
'letter' is the definition.
(Greek letter)
'article's followed by thank-you' is the wordplay.
'article' becomes 'the' (the definite article in English).
'followed by' says to put letters next to each other.
'thank you' becomes 'ta' (informal way of saying thank you).
'the'+'ta'='THETA'
(Other definitions for theta that I've seen before include "Eighth letter of Greek alphabet" , "Letter from Greece" , "Greek letter used in trigonometry" , "Character from overseas" , "8th letter of the Greek alphabet" .)